Your behavior description is a little hard for me to follow... You have master and slave copies on both Redmine servers? I feel like this needs a diagram!

But for some reason you got me wondering if basic duplication of activity across multiple instances could be done through, well, email notifications. Like, server A and server B have their respective email addresses, and are members of each other's projects. A change on server A sends an alert to server B, which parses it using Redmine's native receive-email feature, and it goes to whatever project has the same identifier.